Several processes for the manufacture of 5-cyano-4-lower alkyl-oxazoles have already been described. The synthesis of 5-cyano-4-methyl-oxazole is carried out, for example, by dehydrating 5-carbamoyl-4-methyl-oxazole in the presence of phosphorus pentoxide. The disadvantage of this process is the low yield of product due to carbonization, which occurs very readily in this reaction.
An improvement of this process comprises reacting 5-carbamoyl-4-methyl-oxazole with phosphorus pentoxide in the presence of quinoline as the solvent (U.S. Pat. No. 3,222,374). This process also has disadvantages which result mainly from the use of quinoline, which is toxic, has an unpleasant smell, and is thermally unstable. Also, quinoline is a relatively expensive solvent. A further problem is the regeneration of quinoline. Phosphorus pentoxide also presents problems. It must be used in stoichiometric amounts and its by-products must be worked up and disposed of in an environmentally correct manner.
Another known process (U.S. Pat. No. 4,011,234) for the manufacture of 5-cyano-4-methyl-oxazole comprises reacting 5-carbamoyl-4-methyl-oxazole with a lower alkanecarboxylic acid anhydride and subjecting the reaction mixture or the 4-methyl-5-(N-lower alkanoyl-carbamoyl)-oxazole isolated therefrom to a pyrolysis. However, the final pyrolytic step has certain disadvantages. In particular, corrosion problems with the reactor materials occur and byproducts which are difficult to recycle are formed.
The one-step conversion of 5-ethoxycarbonyl-4-methyl-oxazole into 5-cyano-4-methyl-oxazole is disclosed in U.S. Pat. No. 4,772,718. In this process the oxazole ester is converted into the cyano-oxazole in the presence of ammonia and a zirconium oxide or hafnium oxide catalyst in the gas phase. Disadvantages are the use of the relatively expensive catalyst, and the maintenance of very precise reaction conditions in order to optimize reaction control.
A further process (U.S. Pat. No. 4,026,901) comprises catalytically dehydrating 5-carbamoyl-4-methyl-oxazole to 5-cyano-4-methyl-oxazole while heating in the presence of phosphorus pentoxide on a solid carrier. Handling 5-carbamoyl-4-methyl-oxazole is a disadvantage of this process. Sublimation is a prime consideration, and so is the solid dosing of the low volatility starting material.
The subject invention is a process for the manufacture of 5-cyano-4-lower alkyl-oxazoles which does not have the disadvantages of the state of the art.